Chlamydia trachomatis serovars A-L3 are the causative agents of hyperendemic blinding trachoma, a largely neglected disease of the developing world, and sexually transmitted infections (STI) that are epidemic world wide. Chlamydial STI are risk factors for HIV and a cervical cancer co-factor. Control of these important human diseases is the long term goal of this project. Towards this end our goal is to develop a safe and efficacious vaccine to prevent these diseases. The obligate intracellular life style, complex developmental biology, and antigenic structure of chlamydiae have severally hindered progress in vaccine development. A major hurdle in chlamydial vaccine development has been the unavailability of antigens capable of providing broadly reactive protective immunity against the multiple serovariants (15) that cause oculogenital infections. The immediate goal of this work was to discover antigen(s) with characteristics amenable for the development of a broad based multi-strain vaccine against chlamydial infections and to test the candidates in pre-clinical models of infection and disease.